Plates, films, and proofing media for imaging systems, such as computer-to-plate (CTP) systems which are used in the graphic arts industry, are commonly stacked in boxes with slip sheet sheets interspersed between adjacent media sheets. The slip sheet primarily functions to protect the media surface from damage and to prevent adjacent media sheets from adhering to each other. The slip sheet is particularly important when the active emulsion on the media is either sticky or particularly sensitive to damage.
In the development of media for imaging tasks, particularly plates for lithographic printing operations, a number of often conflicting parameters such print run length, exposure sensitivity, exposure latitude and processing requirements must often be traded off against one another to achieve the best results. Some media have particularly good performance in relation to the abovementioned parameters, but suffer from extreme delicacy of the unexposed media emulsion. In a specific case, some LH-PI lithographic plates have a particularly delicate emulsion and may be scratched even by the action of removing the slip sheet. However, once exposed, the emulsion is durable and the plate images have good on-press performance.
Other commonly available media may have similar problems with delicate emulsion surfaces although, depending on the thickness and particular characteristics of the emulsion, scratches may or may not remain visible after subsequent processing. While not all media require special attention to how slip sheets are removed, the problem has been evident in a number of media products in the graphic arts industry.
In U.S. Pat. No. 5,655,452 (Blake et. al.) a slip sheet removal mechanism for removing a slip sheet from a plate is described. Briefly, the removal operates by activating peeler airflow to initiate separation between the slip sheet and the plate. A number of suction cups are pivoted into a location above the slip sheet and the stack of plates is moved to bring the slip sheet into engagement with the suction cups. A combination of movement of the plate stack and pivoting of the suction cups is used to separate the slip sheet and move it towards a pair of nip rollers that complete the removal operation.
There remains a need for better methods of handling slip sheets and there is a particular need for such a slip sheet removal mechanism that performs the removal without any damage to the media emulsion.